The pathway regulated by Wnt/-catenin plays an important role in nearly all colon cancers. Inheritance of a germline mutation in APC drives the colon cancer syndrome, Familial Adenomatous Polyposis (FAP), and aberrant activation of the -catenin pathway either through mutation of -catenin or more commonly by loss of the APC tumor suppressor gene occurs in almost all spontaneously airing colorectal cancers. Oncogenic activation of -catenin has also been implicated in other cancers such as breast, ovarian, prostate and liver carcinomas. Although many of the components of this signaling pathway are now known, the mechanisms that regulate this pathway and its role in cancer progression remain incompletely understood. In recent work, we have found a connection between amplifications of CDK8, -catenin signaling and colon cancer. As part of a comprehensive effort to identify novel human oncogenes by integrating high throughput functional genomic approaches with experimental models of human cell transformation and on-going structural characterization of cancer genomes, we found that CDK8, a component of the Mediator complex, is amplified and overexpressed in a substantial subset of human colon cancer cell lines and tumors, is required for the proliferation of colon cancer cell lines that harbor CDK8 copy number gain, and regulates for -catenin-dependent transcriptional activity. Forced expression of CDK8 induces cell transformation, and CDK8 kinase activity is necessary for -catenin-dependent induced transformation. These observations identify CDK8 as a colon cancer oncogene that participates in the regulation of WNT/-catenin pathway. Based on these observations, this proposal focuses on investigating the role of CDK8 in colon cancer pathogenesis. Specifically, biochemical, genetic, molecular biological and pharmacologic approaches will be applied to elucidate the role of CDK8 in regulating -catenin, to identify other CDK8 targets that participate in cell transformation and to validate CDK8 as a potential therapeutic target. Investigating the role of CDK8 in colon cancer development will not only enhance our mechanistic understanding of this new oncogene but will also clarify the role of the Mediator complex in the development of human epithelial cancers. In addition, these studies will provide a foundation for strategies to target this kinase oncogene therapeutically.